Objective-C Foundation
Lecture 2
1
Flow of Lecture 2
Objective-C Basics
 Introduction to Objective-C
 Intrinsic Variables and Image-Specific Variables
 Execution Flow Control
Object-Oriented Programming in Objective-C
 Class and Object
 Example Codes for Defining and Using a Class
Objective-C Pointers
 Pointer and Memory Management
 Objective-C Type of String
 Objective-C Type of Array
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Flow of Lecture 2
Objective-C Basics
 Introduction to Objective-C
 Intrinsic Variables and Image-Specific Variables
 Execution Flow Control
Object-Oriented Programming in Objective-C
 Class and Object
 Example Codes for Defining and Using a Class
Objective-C Pointers
 Pointer and Memory Management
 Objective-C Type of String
 Objective-C Type of Array
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Introduction to Objective-C

Objective-C is the official programming language
used to develop iPhone applications

Objective-C is the superset of C
 Everything that works in C also works in
Objective-C
 Implication:
We can use a C style programming in developing
iPhone app although it is not formal
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Intrinsic Variable Types
Intrinsic variable types we use in C/C++ also work in Objective-C
Below is some of the supported intrinsic variables types
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Objective-C Type
int
unsigned
float
double
char
BOOL
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Description
Example
Integer
int anInteger = -10;
Unsigned integer
unsigned anInteger = 50;
Single precision
float myFloat = -0.333;
floating point number
Double precision
double myDouble =
floating point number 0.33827382732234;
Character
char myCharacter = ‘s’;
Boolean variable
BOOL isHappy = 0;
Image-Specific Variable Types
We need image-specific variable type (class) for displaying the
graphics on the screen when creating a GUI application
Below are all those we are using in the project
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Image-Specific
Type (Class)
UIImageView
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Description
Provides a view-based container for displaying a
single image
(e.g. The grey time bar)
UILabel
Implements a read-only text box on the view
(e.g. The “Target Shot” label)
UIButton
Implements a button on the touch screen that
intercepts touch events
(e.g. The “Shoot” button)
Flow Control – Conditional Statement
Objective-C uses exactly the same syntax as that of
C/C++ in conditional statements
Example:
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int a = 0;
int b = 1;
if (a < b){
// Do something here
} else{
// Do another thing here
}
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Flow Control – Loop Control
Objective-C uses exactly the same syntax as that of
C/C++ in for-loop and while-loop statements
For-loop example:
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for (int i = 0; i < b; i++){
// Do something here
}
While-loop example;

while (a < b){
// Do something here
}
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Flow of Lecture 2
Objective-C Basics
 Introduction to Objective-C
 Intrinsic Variables and Image-Specific Variables
 Execution Flow Control
Object-Oriented Programming in Objective-C
 Class and Object
 Example Codes for Defining and Using a Class
Objective-C Pointers
 Pointer and Memory Management
 Objective-C Type of String
 Objective-C Type of Array
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Object-Oriented Programming (OOP)

The programming style that you have learned before is
called procedural programming style
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Break down programming task into functions / sub-routines
The program composes of a sequence of function calls
Object-oriented programming introduces concept of
“objects”
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Break down programming task into objects
The program is modeled as interactions between different
objects by passing of messages (invoking the methods of the
objects)
Each object has its own states (member variables) and
behaviours (responses to method invocations)
Class vs. Object

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We need to define classes in program code before
creating and using objects
A class is a template / prototype of certain kind of
objects

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Basically, it is a type (compare with int, float …)
Defines the characteristics of objects that belong to the class
 Defines the fields (member variables) that the object has
 Defines the things that the object can do (methods)
An object is an instance of a class
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Each object has its own copy of its member variables
Each object can respond to invocation of its methods
Many objects can be created from the same class
Objective-C Object

For example, in a GUI application, a given text label on
the screen is an object (e.g. the “Target Shot” label)
 It belongs to class UILabel
 It has its state:
The text shown on the label
 It has its behaviour:
When receiving a method call (say, telling it to change the
text), it will respond to the caller by changing its own
label
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Example: Bullet Class

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We use an example to show how to declare and define a
class
Example: Bullet
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In our project, the bullets we see on the screen are modeled as
objects
Each bullet has its own center position x, y, and radius
We can invoke method (send message) to operate on a bullet
e.g. Tell it to translate (move) for a certain amount of pixels
When a bullet receives the method call, it will translate itself
according to the given parameters
Objective-C Files: .h and .m

Bullet.h
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The header file that contains the declaration of a class
Also contains declaration of member variables and methods of
the class
We use the “#import” statement to include other .h files (e.g.
Foundation.h)
Bullet.m
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Contains the definition (implementation) of the methods of the
corresponding class, manipulation of data and objects
We need to use the “#import” statement to include the
Bullet.h file
Class Declaration (.h File)

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Lines 1 – 2 are library and header file import statements
#import works like the C/C++ #include statement
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Class Declaration (.h File)
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Line 4 defines the class name “Bullet”
“: NS Object” is about inheritance – ignore it by now
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Class Declaration (.h File)
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Lines 5 – 7 declare the list of member variables
Use “float” because of sub-pixel rendering in iphone app.
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Class Declaration (.h File)

Lines 10 – 12 are needed to get and set the variables of
an object of Bullet class from other objects
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Class Declaration (.h File)

Lines 14 – 16 define the two methods (messages)
supported by this class
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Class Definition (.m File)

Line 1 imports the declaration file (.h)
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Class Definition (.m File)

Line 3 is the standard implementation syntax
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Class Definition (.m File)

Line 4 directly corresponds to the three @property
statements in lines 10 – 12 of the .h file
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Class Definition (.m File)

Line 6 – 12 contain implementation of the first method
initialialze which takes 3 parameters
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Class Definition (.m File)

Lines 14 – 17 contain implementation of the second
method move which takes 2 parameters
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Class Definition (.m File)

Lines 19 – 21 must be included in all classes – related to
object de-allocation
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Syntax of Methods

Method that takes parameters
- (void) move: (float) moveX: (float) moveY {
// Do something here
Parameter
}
Return type
of method
Instance method: -
(type) name
Method name
Class method: +
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Method that takes no parameters
- (int) getValue {
// Do something here
return anInt;
}
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Class Method vs. Instance Method
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Class Method
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A class method is static and associated with a class
Denoted by a “+” sign in method declaration
e.g., in the Bullet class, we can add a method
printClassName that prints the class name on the screen
Invoke class method by [Bullet printClassName];
(use the class name directly)
Since this method does not require association with a
particular object, so printClassName should be a class
method
Class Method vs. Instance Method
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Instance method
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An instance method associates with an object
Denoted by a “-” sign in method declaration
e.g., we call the move method on a particular object by
//create an object called myBullet of type Bullet
[myBullet move: 10.0: 5.0]; (myBullet is
an OBJECT, not the class name)
Since every object can move differently, so move should be an
instance method
Example: Bullet Class (Continued)
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We now need to know how to make use of the Bullet
class, specifically:
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How to create an object of class Bullet during runtime
How to initialize the position and radius of the object
How to invoke method of (send message to) the object
How to get and set the member variables of the object
How to de-allocate the object to free up system memory after
use
Manipulation of Objects

C/C++/Objective-C all have the main function as the entry point of
the program
 “main.m” for Objective-C
 Here, we assume the codes that manipulate objects are placed in the
main.m, like this:
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Bullet Class: Create and Initialize Object
OR


Object creation and initialization can be done altogether
 [Bullet alloc]; creates the object
 [
...
initialize<parameters>];
sets the initial values of the object
 centerX, centerY and radius are float variables already
defined
 Bullet * bullet = [
... ]; assigned the object to the
variable bullet
 NOTE: Bullet is the name of the class; bullet is the variable
name of the object
We should implement an initialization method for all classes
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Bullet Class: Invoke Method (Send Message)

Calling a method
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[bullet ...]; is the variable name of the object
[
...
move<parameters>];
calls the move method with parameters
moveX, and moveY are float variables already defined
If the move method accepts no parameters, then simply write:
[bullet move];
C++ equivalent:
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bullet->move(moveX, moveY);
Bullet Class: Get and Set Variables
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Getting and setting a variable from outside Bullet.m can
only be done when:
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Getting a member variable of a bullet object:
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@property statement for that variable is defined in .h file
@synthesize statement for that variable is defined in .m file
e.g. float x = bullet.centerX;
e.g. float rad = bullet.radius;
Setting a member variable of a bullet object:
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e.g. bullet.centerX = 10.0;
e.g. bullet.centerY = 30.0;
Bullet Class: De-allocate Object
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De-allocating the object
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Simply call [bullet release];
NOTE:We are not calling the dealloc method implemented
in the Bullet class
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This is because the program will automatically call it at appropriate
time to really de-allocate the object
C++ equivalent:
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delete bullet;
Flow of Lecture 2



Objective-C Basics
 Introduction to Objective-C
 Intrinsic Variables and Image-Specific Variables
 Execution Flow Control
Object-Oriented Programming in Objective-C
 Class and Object
 Example Codes for Defining and Using a Class
Objective-C Pointers
 Pointer and Memory Management
 Objective-C Type of String
 Objective-C Type of Array
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Pointers
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Objects are usually created dynamically during runtime
A pointer is used to store the memory address that holds
up the object (“points to” the object)
1. Declaring a pointer variable:
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2. Allocating an object with initial values, and pointing to
that object:
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Bullet * bullet;
bullet = [[Bullet alloc] initialize: 10.0: 10.0: 5.0];
Combining 1 and 2 as a single statement;
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Bullet * bullet = [[Bullet alloc]
initialize: 10.0: 10.0: 5.0];
Pointer Example
Memory (heap space)
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Pointer Example
Memory (heap space)
bulletA
bulletB
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Pointer Example
Memory (heap space)
bulletA
bulletB
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centerX = 5.0;
centerY = 10.0;
radius = 15.0;
Pointer Example
Memory (heap space)
bulletA
bulletB
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centerX = 5.0;
centerY = 10.0;
radius = 15.0;
centerX = 6.0;
centerY = 12.0;
radius = 18.0;
Pointer Example
Memory (heap space)
bulletA
bulletB
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centerX = 7.0;
centerY = 13.0;
radius = 15.0;
centerX = 6.0;
centerY = 12.0;
radius = 18.0;
Pointer Example
Memory (heap space)
bulletA
bulletB
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centerX = 6.0;
centerY = 12.0;
radius = 18.0;
Pointer Example
Memory (heap space)
bulletA
bulletB
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Pointer Example – Memory Leak
Memory (heap space)
bulletA
bulletB
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centerX = 7.0;
centerY = 13.0;
radius = 15.0;
centerX = 6.0;
centerY = 12.0;
radius = 18.0;
Pointer Example – Memory Leak
Memory (heap space)
bulletA
bulletB
Memory leak occurs:
This object can no longer be
accessed but is kept in memory
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centerX = 7.0;
centerY = 13.0;
radius = 15.0;
centerX = 6.0;
centerY = 12.0;
radius = 18.0;
Pointers – Memory Management

We should remove the object when we do not need it in
future
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This avoids memory leaks
Each alloc should be associated with a release
Pointers – The “Self” Reserved Word

Imagine there is a pointer called self which points to
the object itself when an object is created
self
bullet
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Memory (heap space)
centerX = 5.0;
centerY = 10.0;
radius = 15.0;
Pointers – The “Self” Reserved Word

Sometimes, we need to refer to the method or variable
of the bullet object within a method implementation of
the Bullet class
 We
Bullet.h
should use the keyword “self” as the subject
Bullet.m
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Member variables
Parameters
Pointers – The “Self” Reserved Word
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Sometimes, we even need it to call on or return the object itself
Bullet.m
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The purpose is to make this call valid
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The return type of the method initialize is a pointer
The method returns a pointer to itself, which will be pointed to
by the pointer variable bullet
Objective-C String
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Strings in Objective-C are defined with an “@” sign
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There are two types of strings: NSString and NSMutableString
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e.g. @"Carson“
It differentiates Objective-C strings from C style strings
A string of type NSString cannot be modified once it is created
A string of type NSMutableString can be modified
Here, we will cover NSString only
Refer to documentation for NSMutableString
Creation of a string of class NSString:
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e.g. NSString * carsonStr = @"Carson";
carsonStr is a pointer to an NSString object which is
dynamically created
Objective-C String: NSString

Example:
int age = 29;
float gpa = 3.4;
NSString * carsonStr = @"Carson";
NSString * string1 = [NSString stringWithFormat:@"%@
age %d, %@ gpa %f ", carsonStr, age, carsonStr, gpa];
// NSLog is to output string to the Console
NSLog(string1);
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The output on the console is:
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Carson age 29, Carson gpa 3.4
Objective-C Array

To make an array holding elements of intrinsic variable
type (e.g. int)
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We can simply use C/C++ style of creating array
Example:
int myArray[100];
myArray[0] = 555;
myArray[1] = 666;
Objective-C Array

To make an array holding objects of a class (e.g. Bullet)
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Create an array object of class NSArray or NSMutableArray
Add the objects of the same class into the array object
array object
centerX = 5.0;
centerY = 10.0;
radius = 15.0;
centerX = 6.0;
centerY = 12.0;
radius = 18.0;
bullet objects
...
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Finding the size of it is important when we want to
traverse the whole array
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We can do so by invoking the count method
e.g. [array count];
Objective-C Array: NSArray
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NSArray is static and cannot be changed at runtime
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i.e., We have to define all the elements inside the array when we initialize
the array by using the initWithObjects method
The array has to be terminated by “nil”
We can get a particular element inside the array by invoking the
objectAtIndex method
The output is:
array[0] = red
array[1] = white
array[2] = blue
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Objective-C Array: NSMutableArray

By using NSMutableArray, we can dynamically modify the
array elements by using the method addObject
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Objective-C Array: NSMutableArray
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When a particular element inside the array is no longer in
use, we can remove the element from the array
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Release the memory held by an object in the array
 [[array objectAtIndex:i] release];
Remove the element from the array
 [array removeObjectAtIndex:i];
 All elements beyond index i will then be moved one position
forward
Remove the array object itself
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[array release];
A Summary
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Comparison between C/C++ and ObjectiveC
C/C++
Objective-C
Framework inclusion
#include <stdio.h>
#import <Foundation/Foundation.h>
Header File inclusion
#include "Dog.h"
#import "Dog.h"
Constant Definition
#define RATE 1.0
#define RATE 1.0
Header File/ Implementation File
C: .h/.c
C++: .h/.cc
.h/.m
Member Variables Declaration
int age;
int age;
Class Method Declaration
static void helloWorld();
+ (void) helloWorld;
Instance Method Declaration
int getAge();
- (int) getAge;
Dynamic Memory Allocation and
Pointer variable Assignment
Dog * dog1=malloc(sizeof(Dog));
Dog * dog1 = new Dog;
Dog * dog1 = [Dog alloc];
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Comparison between C/C++ and Objective-C
C/C++
Objective-C
Class Method Invoke
Dog::getAge();
[Dog getAge];
Instance Method Invoke
dog1->getAge();
[dog1 getAge];
String Format
"Hello"
@"Hello"
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